I ampretty sure they have spawn in the middle (more or less) of the map, for the aliens. I went in a tent, I looked everywhere inside to see if I could find some loot. The when I was about to go out three alien appeared at the door (which could have come from outside) but I also found myself with two more aliens behind my back which is impossible because I secured the whole tent 2 sec before and no one could have entered because I had the door right in front my eyes.
It gave me the impression that the whole group of aliens teleported on me, I could understand for those outside the tent but not for those inside, they appeared right when I was about to go out, it really gave the impression to me that they spwned on me, it was not like when ennemies enter in your vision range as they weren't moving at all, they started moving after they entered my vision range.
For the two bandits spawn it was on the edge of the map, like you stated before there are ennemy spawn on every map but every time new ennemies spawn on the map they spawn right behind my back, it makes it impossible for me to escape as I am dead the second they spawn.
I could understand that new ennemies constantly spawn until you leave the map, but here it seems that they spawn right next to me and I can't do anything to avoid contact with those groups although I could avoid contact with those already spawned on the map when I entered.